When using tiled resources or sparse textures, an application managed paging system takes an address space that covers a very large texture that cannot be loaded at once. But instead of loading the whole texture, only portions thereof called tiles are loaded. While the address space covers a very large area, only a small part of the address space is actually close to the viewer. Then usually only the smaller mip levels and a tiny part of the larger mip levels are used.
The application decides which chunks of the address space get mapped to memory. Then the system translates from tiled resource address space into physical or virtual address space. As a result, when a page is accessed, it may or may not be mapped to memory. Near an edge, filtering may overlap over into unmapped areas. This may result, for example, in returning zeros and causing the image to fade to black.
With anisotropic filtering, it is hard to know which tiles will be touched. However, using data from unmapped tiles may result in visually perceptible artifacts.
In texture sampling, texel values are specified at the centers of the texel grid. This means that any linearly interpolating sampling operation near the border (i.e. in the outermost half texel-wide region) will access texels both inside and outside the valid texel domain.
This is a problem since the out-of-bounds texel(s) may be ill-defined, i.e., either assigned an incorrect color by wrapping, or belong to a null tile in a tiled resource. Even if sample taps for out-of-bounds texels are discarded, a half texel-wide region of incorrectly interpolated color remains. This is visually distracting and prevents current texture filtering from being used for multi-texturing (Ptex).